A conventional example of fault diagnosis system of sensor function is explained by referring to FIG. 7. An output signal S1 from a drive circuit 1 is input into a sensor 2, and the sensor 2 is driven. The sensor 2 converts a physical quantity into an electric signal, and sends out a sensor output signal S2. The sensor output signal S2 is input into an interface circuit 3. The interface circuit 3 processes the sensor output signal S2 into a signal to be recognized by a 1-chip microcomputer 4, and issues an electric signal (digital signal) S3.
The 1-chip microcomputer 4 converts the electric signal S3 received from the interface circuit 3 into a control signal S4 having a specified function by a program stored in a memory 4A in advance, and issues to an output circuit 5. The output circuit 5 drives a load 6 depending on the drive signal S4.
The sensor output signal S2 usually settles within a certain output range of the sensor output. However, it a function trouble occurs in the sensor 2, and the sensor output signal S2 does not settle within the output range, the electric signal S3 from the interface circuit 3 goes out of the input range of the 1-chip microcomputer 4. Accordingly, the 1-chip microcomputer 4 issues a fail signal S5. When receiving the fail signal S5, the output circuit 5 lights up a fail lamp 7. As a result, if the sensor output signal S2 does not settle within the output range, such trouble of the sensor function can be detected.
However, in spite of the trouble in the sensor function, if the sensor output signal S2 settles within the output range, it is not known what signal is produced when the electric signal S3 is issued from the interface circuit 3, and it is hard to detect fault of the sensor function.
It is hence an object of the invention to present a method and device for fault diagnosis of sensor function capable of detecting trouble of sensor function even if a sensor output signal issued from a sensor settles within a certain output range.